We used a field experiment to assess the individual and combined effects of removing top predators and enriching water column nutrients (nitrogen-N and phosphorus-P) on seagrass ecosystem structure and function. Experiments were conducted in turtlegrass (Thalassia testudinum) habitats in St. Joseph Bay, FL, an aquatic preserve in the northern Gulf of Mexico that exhibits low ambient nutrient concentrations and contains abundant populations of small crustacean and gastropod mesograzers. We stocked 7.0 m 2 enclosures with elevated (ϳ4-8ϫ ambient) densities of juvenile pinfish (Lagodon rhomboides), the dominant fish species in local seagrass habitats, to simulate the first-order effects of large predator reductions, and we used an in situ delivery system to supplement N and P to ϳ3ϫ ambient levels in nutrient addition treatments. Monthly determinations of water column nutrients and Chl a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass, and shoot and leaf densities were used to evaluate the relative effects of manipulating nutrient supply and altering food web structure.In contrast to our expectations, results showed few significant nutrient effects, or fish ϫ nutrient enrichment effects on any of the parameters measured. However, there were many significant fish effects, most of which were unexpected. As predicted, increased pinfish density reduced mesograzer numbers significantly. Not anticipated, however, was the reduced epiphyte biomass in fish enclosure treatments, apparently brought about by the pinfish consuming significant amounts of epiphytes as well as mesograzers. This reduction in epiphyte biomass produced positive indirect effects on seagrass biomass, shoot number, and rates of primary productivity in pinfish enclosure treatments.Our results also showed important top-down effects in determining the composition and abundance of seagrassassociated plants and animals in this pristine environment. Although we did not observe simple trophic cascades, most likely because pinfish fed at more than one trophic level, and because the dense seagrass prevented small grazers from being reduced to low numbers, pinfish produced important changes in the epibiota as well as the seagrasses themselves. These data, while contrasting with studies reporting significant negative nutrient enrichment effects on seagrasses, support the results of recent experimental studies in showing that: (1) small grazers can often control the abundance of epiphytes; and (2) it is unlikely that a full understanding of the consequences of nutrient enrichment for seagrass ecosystems can be gained without knowing how grazer population are regulated.

Isotopic fractionation of ammonium (NH,+ ) and nitrate (NO,-) during uptake by phytoplankton was examined in batch culture experiments with the diatom SkeZetonema costatum under nitrogen-enriched conditions (5-100 PM). The fractionation factor (E) for NOJ-uptake by Skeletonema was -9.Of0.7%0 and was concentration-independent. For NH,+, E was more variable and dependent on ambient NH,+ concentration. For NH4+ concentration ranges of 100-50, 50-20, and 20-5 PM, E was -24.6k5.5, -27.2-+ 1.6, and -7.8+3.0%.In these cultures, isotopic fractionation by phytoplankton caused variations in 615N of up to 50?& for NH4+, 1 ~%XI for NO,-, and 25% for particulate N. Similar variability in the 615N of both dissolved inorganic and particulate organic N pools should be expected during phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions.

Phytoplankton production in the Delaware Estuary (USA) was measured over several seasonal cycles (1980)(1981)(1982)(1983)(1984)(1985). Seasonal variability in daily area1 production (JP; g C m-2 d-l) was dlrectly related to chlorophyll concentrations in the upper estuary, ranging from a maximum of 1.1 g C m-' d -' In summer to a minlmum of d-l) dunng summer in the presence of low phytoplankton biomass (2 to 10 kg Chl I-'), and in mid-estuary [2.6 g C d-l) during the spring diatom bloom (50 to 60 yg Chll-l). Desplte the occurrence of maximum nutnent concentrations in the freshwater region, highest JP and 90 % of the annual production occurred in the lower estuary, down-stream from the turbidity maximum. The presence of the turbidity maximum immediately downstream from major anthropogenic nutrient sources restricts phytoplankton growth, and limits biomass accumulation below nuisance levels. Annual production for the 1981-1985 period averaged 307 g C and displayed marked inter-annual variability. Llght availability is the predominant regulator of production in the estuary. Although growth was light-limited, neither chlorophyll specific produchon nor the light intensity at which photosynthesis saturates was related to the mean light intensity in the mixed surface-layer. These results suggest that photoadaptive response times are slower than the vertical mlxing rate and that photoadaptation is of mlnor significance to overall production in the system.